Innovation in the field of hydraulic controls has continued over the course of many decades as improvements in hydraulic valve configurations have been made to meet particular needs. The prior art includes a variety of rotary-action and/or directional control valves for hydraulic motors of various kinds and serving various purposes.
Some examples of prior hydraulic control valves are those disclosed in the following U.S. patents:
U.S. Pat. No. 3,796,232 (Dalton) PA1 U.S. Pat. No. 4,445,540 (Baron et al.) PA1 U.S. Pat. No. 3,506,706 (Schmitz) PA1 U.S. Pat. No. 3,405,602 (Clarke) PA1 U.S. Pat. No. 4,372,341 (Crawley) PA1 U.S. Pat. No. 2,519,574 (Holl) PA1 U.S. Pat. No. 4,564,045 (Koch et al.) PA1 U.S. Pat. No. 3,083,731 (Hasbany) PA1 U.S. Pat. No. 2,471,285 (Rice) PA1 U.S. Pat. No. 2,622,372 (Moulden).
For a variety of hydraulic applications, flow in both directions through the motor is required. In any situation in which a driven member is moved (typically rotated) in one direction at certain times and in the other direction at other times, a directional hydraulic control valve is needed One example is a hydraulic motor of the type that drives the cutting reels of commercial lawn mowing equipment, which requires hydraulic flow in one direction to turn the reels in a cutting direction and in the reverse direction to sharpen the blades.
In this specific application and many others, it is often highly desirable not only to have a hydraulic control device which reverses the flow but one providing a preset constant drive rate in at least one direction. In some cases, constant drive rates in both directions (although perhaps differing from each other) are highly desirable. Constant drive rates require a substantially constant flow of hydraulic fluid into the hydraulic motor, regardless of changing input pressures and loads.
In the example of the hydraulic motor for driving mowing reels, it is at least desirable to provide a constant and fairly low rate of hydraulic drive (flow) in the reverse (blade-sharpening) direction. A constant flow rate may be less important for the hydraulic drive in the forward direction; in this particular application, it may in some cases be acceptable to allow the forward flow rate to vary widely, depending on input pressure and load, while in other cases it may be desirable to have a constant forward flow as well as a constant reverse flow. In either case, a significantly greater flow is typically desirable in the forward direction.
Hydraulic devices which carry out a number of functions, such as those described above, are typically quite complex and in some cases bulky in construction. There is a need for an improved hydraulic directional control valve which includes pressure-compensating features to provide constant flow in one or both output directions. An improved hydraulic control valve is needed which has these multi-function capabilities, yet is compact, simple in construction and easy to manufacture and assemble.